Multi-purpose portable working devices such as, but not limited to, chain saws, cutting tools and grass trimmers having internal combustion engines are well known. Each of these types of devices has a working assembly, such as a chain or cutting blade, which is brought to an operating rotational speed by the included engine. Since the working assembly is often close-by the operator, there is a risk of contact and an accidental injury occurring. Therefore, such devices are often equipped with a mechanical security brake for the assembly, together with other security arrangements such as requiring active hand-grip engagement by the operator in order to affect operation.
These types of devices are normally equipped with a centrifugal clutch that engages the working assembly when the engine exceeds a certain rotational speed, or clutch-in speed. In normal operation, the clutch improves safety because the tool does not rotate when the engine speed is reduced below the clutch-in speed. The risk for bodily injury is therefore significantly reduced.
These devices are typically started with the throttle valve positioned in a starting position in order to ensure an efficient start-up, this is sometimes referred to as fast idle. Because of the valve position, more air flows into the engine causing the engine rotational speed to immediately increase above the clutch-in speed at which the clutch engages the working assembly. This can present a risk because the operator will not always follow the operator's manual and ignore safety precautions and thus not engage the mechanical brake (or other security arrangements providing the intended protection) at start of the engine. Still further, as the engine speed quickly rises upon starting, the clutch-in speed will be achieved before the operator is ready for the working assembly (for example, a chain blade) to begin operation.